The present invention relates to a method for producing laminated glass such as employed for automobile windshields and other commercial and industrial safety glass applications. In particular, the present invention relates to a method for producing a laminate comprising a sheet of semi-rigid material encapsulated in a thermoplastic interlayer between layers of rigid transparent materials, such as glass or glass substitutes. The present invention further relates to the superior laminate produced thereby.
Various methods are known in the art for producing laminates comprising flexible sheets of rigid transparent material, such as glass, polycarbonate, acrylic plastic, polyester and the like, having interlayers comprising sheets of material, such as polyvinyl butyral, polyurethane and the like. Typical of such methods are those disclosed by Foster, U.S. Pat. No. 3,406,086 at Cols. 5-7; Rieser et al, U.S. Pat. No. 3,808,077, at Cols. 17-18; Shumaker, U.S. Pat. No. 3,933,552; and those discussed by Masuda, U.S. Pat. No. 4,358,329, at Col. 1; and McMaster, U.S. Pat. No. 4,470,858, at Col. 1. Such bonding processes generally involve the application of heat and pressure, such as are available in oil autoclaves. Laminates have found wide application as safety glass due to their strength and preferable shattering characteristics compared to plain or tempered glass.
Among the problems related to the bonding of laminates has been the presence of moisture and air bubbles between laminate layers, and the wrinkling of flexible laminate layers, all of which optically distort the final product. In addition, prior art bonding methods employing oil autoclaves may introduce oil between laminate layers along the edges of the laminate. To eliminate air, moisture and oil, methods for laminating glass and thermoplastic sheet materials have been developed which further employ peripheral evacuation of the laminate assemblies. Such methods are shown by Rieser et al, Shumaker, Curtz et al, U.S. Pat. No. 4,543,283, Dlubak, U.S. Pat. No. 4,642,255 and Rase, U.S. Pat. No. 4,647,327 which variously use vacuum bags or peripheral vacuum chambers to achieve peripheral evacuation. Evacuation of such vacuum bags or peripheral vacuum chambers, serves to draw air and moisture from between the laminate layers, while the bags or chambers effectively create a barrier to oil penetration at the edges of those assemblies.
Nonetheless, the drawback of these bonding methods is that the application of heat and pressure required to bond the laminate layers is both cumbersome and expensive. To avoid this drawback while maintaining strength and other desirable characteristics, other bonding methods occurring at ambient temperatures have been developed in the prior art which involve the use of adhesive layers, such as disclosed by Masuda, LeGrand et al, U.S. Pat. No. 4,683,172, and McMaster. LeGrand et al further teach the use of a thin film of water containing surfactants as a means of preventing the formation of air bubbles between laminate layers. Nonetheless, both strength and optical clarity of laminates, particularly the entrapment of water and air between laminate layers, continue to be of concern in the production of laminates.
Also of general relevance to the present invention are methods for encapsulating liquids or solids within laminate structures. In particular, methods for encapsulating liquid crystal materials between laminates have been developed and are disclosed by IBM Technical Disclosure Bulletin, Vol. 15, No. 2, dated July 1972 and Ferrato, U.S. Pat. No. 4,418,102. In both of these references, an interlayer is comprised of a seal which completely surrounds the liquid crystal material, forming a sealed volume between laminate layers. In the IBM Disclosure, dielectric RF heating or ultrasonic energy is used to seal plastic layers such as polyvinyl chloride to create a volume which may thereafter be filled with liquid crystal material. The plastic layers forming the volume may further be sandwiched between glass plates. In the Ferrato patent, the seal between glass plates is comprised of a thermoset product of resins, requiring the application of heat, that forms a cavity which may be filled with liquid crystal material. Kruger et al, U.S. Pat. No. 4,469,408, and Ishii et al, U.S. Pat. No. 4,553,821 also show full peripheral frames or seals for encapsulating liquid crystal material in a laminate structure. Ishii et al employ heat and a heat sealing agent to seal the laminates and enclose a volume wherein liquid crystal material lies. Thus, in such applications, the liquid crystal material fills a volume between laminates and does not, itself, adhere to other laminate layers to provide structural support.
The problems of optical distortion in a laminate structure due to trapped air, moisture and wrinkling of interlayers becomes of even greater concern when dissimilar solid materials are joined to form an interlayer in a laminate structure. In the Dlubak patent, a method is disclosed for producing a laminated article having an interlayer comprising of fibrous layer encapsulated in a polyvinyl acetal, such as polyvinyl butyral. The interlayer is sandwiched between two glass outerlayers. The fibrous layer is placed well within the peripheral edge of the laminated article, presumably to avoid edge sealing problems, and is, for example, comprised of a woven lace cotton cloth. When placed between two glass outer layers and subjected to heat and pressure, the two sheets of polyvinyl butyral seal together to encapsulate the fibrous layer. At the same time, the assembly is placed in a vacuum bag having a partial vacuum to remove trapped air and moisture between the laminae. The vacuum, high temperature and pressure are maintained for a period of time during which the polyvinyl butyral is caused to flow through the interstices of the woven lace cloth, and no detectable air bubbles are reported to remain. However, because the Dlubak patent relates to a decorative article which is translucent, rather than transparent, optical imperfections, for example due to wrinkling of the fibrous layer, are not as critical as in applications where transparency is required, as with automobile windshields.
Thus, while the above references deal with encapsulating fluids and decorative articles, neither of which are designed to enhance the structural strength of the laminate, none address the encapsulation of semi-rigid sheet materials between laminates to form optically clear laminate structures. Further, while rigid interlayers are shown by Shumaker, the interlayer comprises an entire layer and is sandwiched between other layers, rather than being completely encapsulated.
Semi-rigid sheet materials are broadly defined as those which can be bent and flexed, but in so doing will tend to crease and fold. For purposes of illustration, such materials include, without limitation cardboard sheets; photography; or more pertinent to the present invention, certain plastic sheet materials such as polyester or Aclar.TM. available from Allied Chemical Company. As well, semi-rigid materials include composite sheet materials having semi-rigid properties, for example liquid crystal coatings sandwiched between sheets of polyester, or electroluminescent coatings on electrodes sandwiched between sheets of Aclar.TM..
Further developments in windshield technology have involved the use of peripheral areas inward from the perimeter of the laminate for various materials, such as optically opaque coatings, and electrically conductive materials, for example as shown by Hurst, U.S. Pat. No. 4,744,844.
Thus, a need exists for an effective method of producing optically clear laminates, for example for use as windshields, glass roofs, moon roofs, side glass - any laminated glass part - wherein semi-rigid sheet materials are encapsulated within a laminate layer, but are located inward from the periphery, permitting other laminating materials to be used along peripheral edge areas. A need further exists for a method for encapsulating such materials and thereby provide additional strength to laminate.